1. Field of the Invention
The present invention is directed to a driver circuit for a Light Emitting Diode (LED) display system that can be used, for example, in cellular phones, PDAs, and other similar visual display units. More particularly, this invention is directed to providing driver circuitry to drive a multi-string configuration of different types of LEDs used in display systems.
2. Background Art
Modern day display systems increasingly rely upon LEDs to render a high quality visual display. The applications can include displays for cell phones, personal digital assistants (PDAs), wireless email devices, etc. It is important that the color, tone (hue), contrast, brightness, and other visual parameters remain consistent per the tolerance limits of the desired application and the specifications. To this aim, the driver circuitry used to drive any such LED display system plays a crucial role in terms of controlling the variations in the parameters and providing stability in the operation of the LED grid used in the display.
Depending on the particular application, the display system could have a submodule where a series of flash LEDs are used in parallel with regular back-lighting LEDs. The voltage and current requirements for these two types of LEDs are different and so is their operation. Also, different LEDs will have different forward voltage drops due to variations in manufacturing tolerances. This results in different output voltage requirements across the LED terminals, and therefore a different optical output for each LED if a common voltage driver is applied. Since multiple LEDs are in use in display systems, such a variation in the optical output of the LEDs leads to a degradation in the overall quality of the display images and may lead to failure in operation due to reduction in the life of an LED.
FIG. 1 shows the schematic for a conventional serial LED driver 100 that drives a single string of LEDs 110 containing ‘n’ number of LEDs, where ‘n’ is an integer. Each of the n LEDs 110a to 110n of the single LED string 110 is connected in a series configuration. The cathode of the last LED 110n of the LED string 110 is connected to one of the two terminals of a resistive component RB 112 at a node 124. The second terminal of the resistive component RB 112 is connected to a ground 128. Since the member LEDs 110a-110n of the LED string 110 are connected in a series configuration, there is a voltage drop as one moves along the electrical path connecting nodes 123 and 124. As the number of LEDs increases in the LED string 110, the voltage drop across the nodes 123 and 124 increases. Accordingly, the output voltage VOUT 108 that is required to drive the single LED string 110 will increase as the number of LEDs increases. For example, assuming a voltage drop of 3-4 volts per LED, an input voltage VIN 102 of approximately 30 Volts would be needed to drive an LED string 110 with 8 LEDs. If the input voltage VIN 102 is powered from a battery, it can be as low as 2.7 Volts for a Li-ion battery and even lower for a single cell or a two cell alkaline battery, which is the common energy source for most displays used in mobile devices. Boosting from such low input voltages to 30 Volts is not very efficient and not feasible since the duty cycle required for such a process approaches 100%.
The feedback loop 122 is a voltage feedback loop that includes the control loop 117 and the FET 116 to regulate the voltage Vout 108 that supplies the LED string 110. The feedback loop 122 measures the voltage across the resistor 112 at node 124 and controls the FET 116 to drive the voltage at node 124 to equal the reference voltage 102. In other words, the control loop 117 compares voltage at node 124 to the reference voltage 102, and controls the on-off duty-cycle of the FET 116 to increase or decrease the output voltage 108 so as to drive the voltage at the node 124 to be equal to the reference voltage 102. The feedback loop 122 operates so as to time average the output voltage 108 by controlling the on-off duty-cycle of the FET 116. Finally, the Schottky diode 106 prevents any reverse current flow from the charge stored on the capacitor 114.
The conventional serial LED driver 100 is undesirable if all the member LEDs 1101-110n of the single LED string 110 are not of the same type. For example, if some of the LEDs are used for back-lighting and others are used for flash (flash LEDs), due to the series configuration, the same current will flow through all of them. However, flash LEDs need a higher current than that needed by LEDs for back-lighting purposes.
FIG. 2 shows another conventional LED driver 200 that is used to drive multiple parallel LED strings 210a-n that are terminated in corresponding resistors 216a-216n. These strings can be part of a main display, a sub display, a flash LED or a key pad LED, among many other things. Each of the LED strings 210 are parallel connected with each, but the LEDs in a particular string are series connected with each other. Therefore, the same current flows through each LED in a particular string. The feedback loop 122 provides a voltage feedback path to control the Vout 208, similar to that described in FIG. 1. More specifically, the control loop 117 measures the voltage at the midpoint 218 of the resistor divider 220, and controls the FET 116 to drive Vout 208 so that the midpoint 218 is equal to Vref 102.
The conventional serial LED driver 200 has poor performance if all the LED strings 210 do not have the same voltage/current characteristics. Since there is no individual current regulation for the each LED string, then the LED brightness from one string to another will vary if the LEDs are not matched. As the forward voltage of the LEDs in each of the parallel LED strings 210 changes, the current flowing in them also changes. Accordingly, there is a variation in the brightness or the optical output of the display system. The variation in forward voltage of the LEDs can be attributed, amongst many other factors, to temperature variations or manufacturing mismatches. Further, different types of LEDs require different voltage drops, for example, flash LEDs have different voltage drop requirements when compared to other LEDs.
In addition, the LED current matching in the parallel LED strings 210 is not guaranteed and depends on the forward voltages of the individual LEDs. Such a current mismatch again leads to a degradation in the output of the display. In other words, the LED driver 200 does not have any method to regulate the current in the individual LED strings, and thus falls short of attaining maximum optical output efficiency of the display system.
Additionally, the LED driver 200 is not power efficient. The output voltage VOUT 208 needs to be set to drive the LEDs with the largest forward voltage drop. If the output voltage VOUT 208 is less than the largest forward voltage drop, the whole string containing that particular LED will not light up. For example, if the maximum expected LED forward voltage drop is 4 Volts, then to drive 4 LEDs in series, the output voltage VOUT 208 needs to be set higher than 4×4 Volts=16 Volts. However, if one of the LED strings only requires 3 volts/per LED for a total of 12 Volts, then the extra 4 Volts is dissipated across one of the resistors 216a-n, which is an efficiency loss of 25% (4 Volts/16 Volts*100).
In view of the foregoing, there is a need for a low cost LED driver for display systems which overcomes the problems associated with the fluctuations in current and voltage for each of the strings of the LEDs and the concomitant fluctuations and inconsistencies in the optical output of the display systems.